Considerable evidence accrued over the last two decades have convincingly implicated decreased serotonin transporter (SERT) binding in the pathophysiology of major depression and suicide. Although a tremendous amount of information regarding the SERT and its role in depression and suicide has been obtained, this supporting evidence has, however, been indirect in being derived almost exclusively from the study of postmortem tissue and animal and peripheral cell models of transporter cell function and pharmacology. The mechanisms responsible for decreased brain SERT binding in depression and suicide are still unknown. We propose to develop fluorine-18 and bromine-76 imaging agents as tools to the exploration of the mechanism of major depression by assessing the functional status of SERT using Positron Emission Tomography (PET). This application wide focus on the development of positron emitting analogs of 2beta-carbomethoxy-3 beta-(4'-(Z-2- iodoethenyl)phenyl)nortropane (ZIENT) that show high selectivity and affinity for the SERT and low nonspecific binding. Our recent preliminary data with iodine-123 labeled ZIENT indicate that structural features unique to ZIENT permit in vivo measurement of SERT density in cortical brain regions suggesting ZIENT is a suitable lead compound for PET radiotracer development. Two goals of this proposal are: 1) conduct a structure activity relationship study (SAR) to identify a sutiable fluorine-18 and bromine-76 radiotracer for in vivo imaging of the SERT in atients with mood disorders; the SAR study will involve: (a) the development of synthetic methods for the preparation of new 18F- and 76Br-labeled 2beta-carboalkoxy-3beta-(4'-(Z-2-haloethenyl)phenyl)nortropanes, (b) the in vitro determination of relative affinity of 18F- and 76Br- candidates (c) ex vivo estimation of brain penetrance and regional distribution in rats. (d) microPET studies defining time- activity curves for specific and nonspecific binding in brain regions-of-interest in rhesus monkeys. (e) ligand metabolite studies performed on arterial samples (f ) microPET studies of in vivo binding site selectivity assessed by "chase" competition in rhesus monkeys. (g) Graphical analysis of cerebral SERT site binding using microPET in non-human primates with the lead radiolabeled analog; 2) To conduct toxicity studies on an optimized ligand that will support the submission of an RDRC and an IND application. Our hypotheses include: 1) the introduction of fluorine onto an alkyl group in the 2P-carboakoxy- position of ZIENT and replacement of iodine with bromine on the 3beta-(4'-(Z-2-haloethenyl)phenyl) group will generate a PET ligand with imaging characteristics equivalent to ZIENT; 2) A 18F- and 76Br-analog of ZIENT provides substantial temporal and logistical benefits for SERT imaging as compared to a SERT [11C] radiotracer due to the longer half-life of fluorine-18 (t1/2=110 min) and bromine-76 (t 1/2= 16 h) compared to carbon (tlI2=20min). After the best novel PET SERT ligand is identified, it will be applied in humans to define the relationship between brain SERT binding and depression and suicidal behavior. We will also begin to define SERT alterations in dimensional traits that are related to suicide, such as aggression, impulsivity and anxiety. The relationship between SERT 5-HTTLPR promoter region and in vivo SERT expression will be explored. Once SERT binding is determined in depressed patients, we will be able to assess the relationship of SERT binding and treatment response to SSRIs.